Timing signal generators are known. They comprise an oscillator for providing a timing signal. The oscillator often comprises a quartz crystal resonator used to stabilize the oscillation frequency. While in principle quartz crystal oscillators are extremely accurate, it is known that their accuracy is detrimentally affected by temperature. A quartz crystal basically acts like a mechanical resonator, and any change in the temperature will cause it to expand or contract ever so slightly, thus changing the resonant frequency. In order to overcome the problems of variations in the resonant frequency, several approaches are known from the prior art.
FIG. 1 is a functional diagram of a prior art timepiece comprising a quartz crystal controlled oscillator 1, a series of binary dividers (flip-flops) 2 and a stepping motor 3 arranged to drive display means 4 of the timepiece in the form of watch-hands. In such a timepiece, the quartz crystal is often a 32′768 Hz quartz crystal tuning-fork resonator. 32′768 equals 215. Accordingly, the dividing chain can comprise fifteen binary dividers, so that the output frequency of the chain is 1 Hz, suitable for driving the stepping motor 3.
32′768 Hz quartz crystal tuning-fork resonators are usually cut in such a way that when frequency is plotted over temperature, it defines a parabolic curve centered around 25° C. In other words, a quartz crystal tuning-fork resonator will resonate close to its nominal frequency at room temperature, but will slow down when the temperature either increases or decreases from room temperature. A common parabolic coefficient for a 32′768 Hz tuning-fork resonator is −0.04 ppm/° C2.
Timepieces equipped with a temperature sensor and capable of compensating for temperature changes are known. Patent document U.S. Pat. No. 3,895,486 describes a temperature compensated time-keeping device, as well as a temperature compensation method. This particular method known as inhibition compensation is used to lower the frequency of a timing signal. For implementing this method, the quartz crystal resonator must deliberately be made to run somewhat fast. Pulse Inhibition compensation consists in having the division chain skip a small number of cycles at regular intervals such as 10 seconds or a minute. The number of cycles to skip each time depends on the temperature and is determined by means of a programmed look-up table.
Another known method for compensating for temperature changes is pulse injection compensation. Contrarily to inhibition compensation, injection compensation works by increasing the frequency of a timing signal. As explained in patent document U.S. Pat. No. 3,978,650 for example, injection compensation consists in incorporating (injecting) additional corrective pulses into the digital signal fed through the chain of binary dividers. Again, the number of pulses to inject is determined by means of a temperature sensor and a programmed look-up table.
Both inhibition compensation and injection compensation are associated with a quantification error. The quantification error stems from the fact that it is not possible to add or suppress only a fraction of a pulse. Quantification limits the resolution to 1/f over 1 second when the frequency of the oscillator is f. If the oscillation frequency of the resonator is f=32768 Hz, the resolution is no better than 30.5 ppm, yielding an error of approximately ±15 ppm. In order to obtain a resolution of 1 ppm for example, it is necessary to compensate over at least 1 million cycles. In the case of a 32768 Hz resonator, this implies waiting at least 31 seconds before applying inhibition or injection compensation. Accordingly, with this type of compensation, the frequency of the 1 Hz output from the chain of binary dividers 2 tends to deviate slightly from its nominal frequency up to the 30th pulse, and the accumulated error is compensated as a whole at the 31st pulse. This is not a problem with a watch, which is a time integrating instrument. However, in the case, for example, of a timing signal generator, the accuracy of each individual pulse should be better than 1 ppm. In this case, the temperature compensation methods described above are not satisfactory. It is therefore an object of the present invention to provide a signal generator in which each individual oscillation is thermally compensated.